Some Points - Anatomy of Eye

a. Name each of the numbered bones.
b. Which bone is thinnest?
c. Which is most likely to fracture after blunt injury?
d. Which is most likely to erode from sinus infections?

Ans. a.The bones are: (1)Sphenoid (2)Zygomatic (3)Maxilla (4)Lacrimal (5)Ethmoid (6)Frontal.
b & c. The ethmoid is the thinnest bone and most likely to perforate from an eroding sinus infection (this happens mostly in kids).
d. The maxillary floor is most likely to fracture from blunt injury.(blow out fracture)

2. What is the uvea? What eye structures compose it?

Ans. The uvea comprises the iris, ciliary body, and the choroid. They are all connected to each other and are histologically similar.

3. Where does the retina get its nutrition supply?

Ans. The inner 2/3rds of the retina (inner implies toward the center of the eyeball) gets its nutrition from the retinal vessels. The outer 1/3 (which includes the photoreceptors) is nourished by the underlying choroid plexus. A retinal detachment, which separates the retina from the choroid, is particularly dangerous for the photoreceptors. This is especially true for detachments involving the macula as the thin macula gets its blood supply primarily from the underlying choroid.

4. Which extraocular muscle doesn’t originate at the orbital apex?

Ans. Unlike the other muscles, the inferior oblique originates from the orbital floor before inserting on the back of the globe near the macula.

5. Which full-thickness eyelid laceration is more dangerous - medial or lateral lacerations? Why?

You worry about the canalicular tear-drainage system involvement with medial lacerations. You want to repair this system as soon as possible, to avoid chronic epiphora.

6. How many layers are there in the cornea? Can you name them?

Ans. There are five: the superficial [A] Epithelium, [B] Bowman’s layer, [C-CENTRAL] Stroma, [D] Decemet’s membrane, and the inner [E] Endothelium.Can be remembered by mnemonic ABCDE

Important Functions of Corneal Layers:

1. Epithelium Layer: This is the surface layer of cells. They provide barrier function and a smooth surface for the tear film.
2. Bowmans Layer: A tough layer of basement membrane right under the epithelium. This layer is tough, and keeps the cornea from swelling forward … which means when the cornea swells, it must do so backwards into the anterior chamber.
3. Stroma: composed of highly arranged collagen fibers and supporting keratocytes, the cornea stroma provides the majority of the cornea. Injuries at this level can scar.
4. Descemet’s Layer: An inner layer of basement membrane. This layer is important for the health of endothelial cells. One of the leading needs for cornea transplant is from a dystrophy of Descemet’s layer called Fuch’s dystrophy.
5. Endothelium: The Endothelium is a crucial layer as it works as a barrier and a pump that keeps the cornea from getting too wet.

Most of these layers are easy to remember: especially the epithelium, stroma, and endothelium. To help you remember the location of the other layers: remember that Descemet’s is Deep, while Bowman’s is up in the BellTower. 

7. How does the water content of the cornea differ from the rest of the eye?

Ans. The cornea is relatively dehydrated, which helps with clarity. If water gets into the cornea, via a disrupted endothelium or a high pressure gradient from acute glaucoma, the cornea turns hazy and white.

8. A pseudophakic (i.e. implanted lens) patient is found to have excellent far vision, but reading is terrible. What’s going on?

 Ans. As we get older, our natural lenses harden and do not change shape very well making it hard to accommodate and see near objects. This phenomenon is called presbyopia and is a normal finding in people over 40 years of age. A prosthetic lens is not able to change shape at all, so all patients (including small children) with implanted plastic lenses require reading glasses to read.

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